Power control for hair iron having ceramic heaters

ABSTRACT

A hair iron includes two longitudinally extending arms each having a ceramic heater. Users place hair between the heaters during use. A controller coordinates timing and voltage conditions to heat the heaters to avoid light flicker and power harmonics. Thermistors provide current heater temperatures of the heaters and a controller calculates a desired heating power. The heating power includes application to the heaters of various half-cycles of AC power from an external AC line voltage.

BACKGROUND 1. Field of the Invention

The present disclosure relates to alternating current (AC) power controlsystems. More particularly it relates to power control methods andapparatus for controlling the AC power delivered to a hair iron havingceramic heaters.

2. Description of Related Art

Many conventional hair irons, such as flat irons, straightening irons,curling irons, crimping irons, etc., suffer from heat lag, which resultsin lengthy times to heat irons for use and cool them afterwards. Duringthese times, users often set their irons on countertops or the likewhich creates a safety risk, especially to children, who mayaccidentally contact them while hot. Additional safety risks also arisewhen users accidentally leave irons powered on when not in use. Theinventors recognize a need for hair irons having faster heating andcooling, which tends to improve safety.

Hair irons draw power from an electrical power grid to operate, i.e.,alternating current (AC) line power. In various geographies, countriessupply power at a relatively low voltage, e.g., 115 VAC, or highvoltage, e.g., 230 VAC. Manufacturers design their irons to operate atone voltage or the other, but not both. When traveling betweencountries, users regularly carry with them voltage-conversion devices toappropriately change the voltage of the line power to match that oftheir hair iron. At either low or high voltage, however, irons can drawpower in such a manner that they generate power harmonics and voltageflicker. In most locations, regulatory bodies set strict certificationrequirements that dictate (un)acceptable amounts of flicker, harmonics,current symmetry, radiation, conduction, and the like to reduceundesirable health effects on users and/or disruption to sensitiveelectronic/electrical equipment. As a result, manufacturers arecontinuingly challenged to meet requirements while not compromisingtemperature control performance and safety. The inventors furtherrecognize a need for hair irons that users can conveniently use anywherewithout requiring voltage converters, while also complying with flickerand harmonics requirements, such as those set by the InternationalElectrotechnical Commission (IEC).

SUMMARY

A hair iron includes two longitudinally extending arms each having aceramic heater. Users place hair between the heaters for heating andstyling. A controller coordinates timing to avoid power flicker andharmonics. Thermistors provide current heater temperatures to thecontroller, whereby desired temperature responses get calculated. Theresponses include application to the heaters of various half-cycles ofAC power from an external AC line voltage.

In other embodiments, a hair iron includes a first arm and a second armmovable relative to each other between an open position and a closedposition. A distal segment of the first arm is spaced from a distalsegment of the second arm in the open position. The distal segment ofthe first arm is positioned in close proximity to the distal segment ofthe second arm in the closed position. A contact surface is positionedon an exterior, such as an exterior of the distal segment, of the firstarm for contacting hair during use. The first arm includes a heaterhaving a ceramic substrate and an electrically resistive trace on theceramic substrate, e.g., on an exterior face of the ceramic substrate.The electrically resistive trace is composed of an electrical resistormaterial. In some embodiments, the electrically resistive trace includesthe electrical resistor material thick film printed on the exterior faceof the ceramic substrate after firing of the ceramic substrate. Theheater is positioned to supply heat generated by applying an electriccurrent to the electrically resistive trace to the contact surface.Embodiments further include those wherein the heater includes one ormore glass layers on the exterior face of the ceramic substrate thatcover the electrically resistive trace for electrically insulating theelectrically resistive trace.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description, isbetter understood when read in conjunction with the appended drawings.However, the invention is not limited to the specific methods andcomponents disclosed herein. Like numerals represent like features inthe drawings. In the views:

FIGS. 1A and 1B show a representative hair iron having power control forceramic heaters;

FIGS. 2A and 2B are views similar to FIGS. 1A and 1B having coversremoved to reveal electronic components;

FIG. 3 is a diagrammatic schematic view of a power control system of ahair iron;

FIGS. 4A and 4B are plan views of an inner face and an outer face,respectively, of a ceramic heater for use in a hair iron;

FIG. 5 is a cross-sectional view of the heater shown in FIGS. 4A and 4Btaken along line 5-5 in FIG. 4A;

FIGS. 6A and 6B show methods of detecting AC line voltage and operatinga hair iron therefrom;

FIGS. 7A and 7B show example waveforms illustrating changes in thehalf-cycles of AC line voltage and power control therefor;

FIG. 8 shows a Table for applying AC half-cycles of power to heaters ofa hair iron to achieve desired temperature responses; and

FIGS. 9A and 9B show further AC waveforms illustrating changes in thehalf-cycles of AC line voltage and power control therefor to have no DCoffset applied to the heaters.

DETAILED DESCRIPTION

It is to be understood that while the preferred embodiments hereinincorporate AC power delivery for a hair iron, the principles andconcepts can be utilized in many other applications. Applications thatare especially well adapted for using the features of the inventioninclude other small appliances where AC power is to be delivered to theappliance at different AC voltages and likely to cause flicker and thegeneration of harmonic energy. The features of the invention can beutilized with AC power systems having frequencies and voltages differentfrom that used in the United States.

Referring now to the drawings and particularly to FIGS. 1A and 1B, ahair iron 100 is shown according to an example embodiment. Hair iron 100includes an appliance such as a flat iron, straightening iron, curlingiron, crimping iron, or other similar device that applies heat andpressure to hair in order to change the structure or appearance of thehair. Hair iron 100 has a housing 102 that forms the overall supportstructure of hair iron 100. The housing 102 may be composed of, forexample, a plastic that is thermally insulative and electricallyinsulative and that possesses relatively high heat resistivity anddimensional stability and low thermal mass. Example plastics includepolybutylene terephthalate (PBT) plastics, polycarbonate/acrylonitrilebutadiene styrene (PC/ABS) plastics, polyethylene terephthalate (PET)plastics, including glass-filled versions of each. In addition toforming the overall support structure of hair iron 100, the housing 102also provides electrical insulation and thermal insulation in order toprovide a safe surface for the user to contact and hold during operationof hair iron 100.

Hair iron 100 further includes a pair of longitudinally extending arms104, 106 that are movable between an open and closed position. Distalsegments 108, 110 of arms 104, 106 are spaced apart from each other inthe open position and are in contact, or close proximity with oneanother in the closed position. The arms clamshell or are pivotablerelative to each other about a pivot axis 112 between the open positionand the closed positions. Hair iron 100 may include a bias member (notshown), such as one or more springs, that biases one or both of arms104, 106 toward the open position such that user actuation is requiredto overcome the bias applied to arms 104, 106 to bring arms 104, 106together to the closed position. A lock 113 is provided to secure thearms in the closed position upon user manipulation.

Hair iron 100 includes a heater positioned on an inner side 114, 116 ofone or both of arms 104, 106. Inner sides 114, 116 of arms 104, 106include the portions of arms 104, 106 that face each other when arms104, 106 are in the open and closed positions. In the example embodimentillustrated, each arm 104, 106 includes a respective heater 130, 132opposed to one another on or within the arm 104, 106. Heaters 130, 132supply heat to respective contact surfaces 118, 120 on arms 104, 106.Each contact surface 118, 120 is positioned on inner side 114, 116 ofdistal segment 108, 110 of the corresponding arm 104, 106. Contactsurfaces 118, 120 may be formed directly by a surface of each heater130, 132 or formed by a material covering each heater 130, 132, such asa shield or sleeve. Contact surfaces 118, 120 are positioned to directlycontact and transfer heat to hair upon a user positioning hair betweenarms 104, 106 during use. Contact surfaces 118, 120 are positioned tomate against one another in a relatively flat orientation when arms 104,106 are in the closed position in order to maximize the surface areaavailable for contacting hair.

With reference to FIGS. 2A and 2B, hair iron 100′ (as seen in partialdisassembled form) includes control circuitry 122 configured to controlthe power, thus the temperature, of each heater 130, 132. The controlcircuitry 122 in this design is bifurcated in the two arms of the hairiron, including a printed circuit board 133, 135 having a front (-f) andback (-b) sides. The PCB boards 133, 135 respectively relate toelectrical circuit components for a power supply unit (PSU) and amicrochip unit (MCU) that coordinate to selectively apply electricalcurrent to the heaters 130, 132 (shown schematically in FIG. 3 ). Thehair iron 100 further includes a power cord 124 for connecting hair iron100 to an external line power or voltage source 126 to power the controlcircuitry 122 and heaters 130, 132. Amongst different geographies, theline power 126 is typically 115 VAC or 230 VAC. Regulating this powerwill be described below in relation to FIGS. 7A and 7B.

With reference to FIG. 3 , the regulation of line power 126 to theheaters 130, 132, includes control circuitry 122. A potentiometer 123receives input from a user by twisting a handle 125 (FIG. 1A) of hairiron 100. The settings are varied, but twists of the handle generallyrelate to the hair iron 100 being off or powered on with high, medium,or low heat to the heaters 130, 132. A corresponding light emittingdiode (LED) 125 indicates to the user whether or not the hair iron 100is powered on or off. Other settings are possible. One or more triacs orswitches 127 connect the heaters 130, 132 to line power 126 undercontrol of the MCU 135. The MCU turns on the triacs 127 when the ACvoltage of the line power is at or near a zero-crossing (ZC) as providedon a zero-crossing detection circuit supplied at 129 to the MCU. Anaccelerometer 131 detects manipulation of the hair iron 100 and the MCUwill stop heating of the heaters 130, 132 regardless of thepotentiometer 123 setting if the MCU does not receive any interrupts 133per a given period of time, say every 60 seconds. In this way, thecontroller knows that a user is manipulating the hair iron for use andnot merely setting it aside. The thermistors 172 gather currenttemperature readings of the heaters 130, 132 that the MCU uses tocontrol the set-points, temperature increases, temperature decreases,and the like, of the heaters. The thermistors provided input to the MCUper a given period of time, such as per every 1 msec. Line voltagevaries per geography, e.g., 115 VAC or 230 VAC, and such is detected bythe PSU 133 and supplied to the MCU for controlling the power to theheaters based thereon as described below in FIGS. 7A and 7B.

In FIGS. 4A and 4B, heaters 130 or 132 are detailed to show them asremoved from their housing. They may or may not be identical to oneanother. FIG. 4A shows inner face 151 of heater 130/132, and FIG. 4Bshows outer face 150 of heater 130/132. In the embodiment illustrated,outer face 150 and inner face 151 are bordered by four sides or edges152, 153, 154, 155 each having a smaller surface area than outer face150 and inner face 151. In this embodiment, heater 130/132 includes alongitudinal dimension 156 that extends from edge 152 to edge 153 and alateral dimension 157 that extends from edge 154 to edge 155. Heater130/132 also includes an overall thickness 158 (FIG. 5 ) measured fromouter face 150 to inner face 151.

Heater 130/132 includes one or more layers of a ceramic substrate 160,such as aluminum oxide (e.g., commercially available 96% aluminum oxideceramic). Where heater 130/132 includes a single layer of ceramicsubstrate 160, a thickness of ceramic substrate 160 may range from, forexample, 0.5 mm to 1.5 mm, such as 1.0 mm. Where heater 130 includesmultiple layers of ceramic substrate 160, each layer may have athickness ranging from, for example, 0.5 mm to 1.0 mm, such as 0.635 mm.In some embodiments, a length of ceramic substrate along longitudinaldimension 156 may range from, for example, 80 mm to 120 mm. In someembodiments, a width of ceramic substrate 160 along lateral dimension157 may range from, for example, 15 mm to 24 mm, such as 17 mm or 22.2mm. Ceramic substrate 160 includes an outer face 162 that is orientedtoward outer face 150 of heater 130/132 and an inner face 163 that isoriented toward inner face 151 of heater 130/132. Outer face 162 andinner face 163 of ceramic substrate 160 are positioned on exteriorportions of ceramic substrate 160 such that if more than one layer ofceramic substrate 160 is used, outer face 162 and inner face 163 arepositioned on opposed external faces of the ceramic substrate 160 ratherthan on interior or intermediate layers of ceramic substrate 160.

In the example embodiment illustrated, outer face 150 of heater 130/132is formed by outer face 162 of ceramic substrate 160 as shown in FIG.4B. In this embodiment, inner face 163 of ceramic substrate 160 includesa series of one or more electrically resistive traces 164 andelectrically conductive traces 166 positioned thereon. Resistive traces164 include a suitable electrical resistor material such as, forexample, silver palladium (e.g., blended 70/30 silver palladium).Conductive traces 166 include a suitable electrical conductor materialsuch as, for example, silver platinum. In the embodiment illustrated,resistive traces 164 and conductive traces 166 are applied to ceramicsubstrate 160 by way of thick film printing. For example, resistivetraces 164 may include a resistor paste having a thickness of 10-13microns when applied to ceramic substrate 160, and conductive traces 166may include a conductor paste having a thickness of 9-15 microns whenapplied to ceramic substrate 160. Resistive traces 164 form the heatingelement of heater 130 and conductive traces 166 provide electricalconnections to and between resistive traces 164 in order to supply anelectrical current to each resistive trace 164 to generate heat.

In the example embodiment illustrated, heater 130/132 includes a pair ofresistive traces 164 a, 164 b that extend substantially parallel to eachother (and substantially parallel to edges 154, 155) along longitudinaldimension 156 of heater 130. Heater 130 also includes a pair ofconductive traces 166 a, 166 b that each form a respective terminal 168a, 168 b of heater 130. Cables or wires 170 a, 170 b are connected toterminals 168 a, 168 b in order to electrically connect resistive traces164 and conductive traces 166 to, for example, control circuitry 122 andvoltage source 126 in order to selectively close the circuit formed byresistive traces 164 and conductive traces 166 to generate heat.Conductive trace 166 a directly contacts resistive trace 164 a, andconductive trace 166 b directly contacts resistive trace 164 b.Conductive traces 166 a, 166 b are both positioned adjacent to edge 152in the example embodiment illustrated, but conductive traces 166 a, 166b may be positioned in other suitable locations on ceramic substrate 160as desired. In this embodiment, heater 130/132 includes a thirdconductive trace 166 c that electrically connects resistive trace 164 ato resistive trace 164 b. Portions of resistive traces 164 a, 164 bobscured beneath conductive traces 166 a, 166 b, 166 c in FIG. 4A areshown in dotted line. In this embodiment, current input to heater130/132 at, for example, terminal 168 a by way of conductive trace 166 apasses through, in order, resistive trace 164 a, conductive trace 166 c,resistive trace 164 b, and conductive trace 164 b where it is outputfrom heater 130 at terminal 168 b. Current input to heater 130 atterminal 168 b travels in reverse along the same path.

In some embodiments, heater 130/132 includes a thermistor 172 positionedin close proximity to a surface of heater 130/132 in order to providefeedback regarding the current temperature of heater 130/132 to controlcircuitry 122. In some embodiments, thermistor 172 is positioned oninner face 163 of ceramic substrate 160. In the example embodimentillustrated, thermistor 172 is welded directly to inner face 163 ofceramic substrate 160. In this embodiment, heater 130/132 also includesa pair of conductive traces 174 a, 174 b that are each electricallyconnected to a respective terminal of thermistor 172 and that each forma respective terminal 176 a, 176 b. Cables or wires 178 a, 178 b areconnected to terminals 176 a, 176 b in order to electrically connectthermistor 172 to, for example, control circuitry 122 in order toprovide closed loop control of heater 130. In the embodimentillustrated, thermistor 172 is positioned at a central location of innerface 163 of ceramic substrate 160, between resistive traces 164 a, 164 band midway from edge 152 to edge 153. In this embodiment, conductivetraces 174 a, 174 b are also positioned between resistive traces 164 a,164 b with conductive trace 174 a positioned toward edge 152 fromthermistor 172 and conductive trace 174 b positioned toward edge 153from thermistor 172. However, thermistor 172 and its correspondingconductive traces 174 a, 174 b may be positioned in other suitablelocations on ceramic substrate 160 so long as they do not interfere withthe positioning of resistive traces 164 and conductive traces 166.

FIG. 5 is a cross-sectional view of heater 130/132 taken along line 5-5in FIG. 4A. With reference to FIGS. 4A, 4B and 5 , in the embodimentillustrated, heater 130/132 includes one or more layers of printed glass180 on inner face 163 of ceramic substrate 160. In the embodimentillustrated, glass 180 covers resistive traces 164 a, 164 b, conductivetrace 166 c, and portions of conductive traces 166 a, 166 b in order toelectrically insulate such features to prevent electric shock or arcing.The borders of glass layer 180 are shown in dashed line in FIG. 4A. Inthis embodiment, glass 180 does not cover thermistor 172 or conductivetraces 174 a, 174 b because the relatively low voltage applied to suchfeatures presents a lower risk of electric shock or arcing. An overallthickness of glass 180 may range from, for example, 70-80 microns. FIG.5 shows glass 180 covering resistive traces 164 a, 164 b and adjacentportions of ceramic substrate 160 such that glass 180 forms the majorityof inner face 151 of heater 130/132. Outer face 162 of ceramic substrate160 is shown forming outer face 150 of heater 130/132 as discussedabove. Conductive trace 166 c, which is obscured from view in FIG. 5 byportions of glass 180, is shown in dotted line. FIG. 5 depicts a singlelayer of ceramic substrate 160. However, ceramic substrate 160 mayinclude multiple layers as depicted by dashed line 182 in FIG. 5 .

Heater 130/132 may be constructed by way of thick film printing. Forexample, in one embodiment, resistive traces 164 are printed on fired(not green state) ceramic substrate 160, which includes selectivelyapplying a paste containing resistor material to ceramic substrate 160through a patterned mesh screen with a squeegee or the like. The printedresistor is then allowed to settle on ceramic substrate 160 at roomtemperature. The ceramic substrate 160 having the printed resistor isthen heated at, for example, approximately 140-160 degrees Celsius for atotal of approximately 30 minutes, including approximately 10-15 minutesat peak temperature and the remaining time ramping up to and down fromthe peak temperature, in order to dry the resistor paste and totemporarily fix resistive traces 164 in position. The ceramic substrate160 having temporary resistive traces 164 is then heated at, forexample, approximately 850 degrees Celsius for a total of approximatelyone hour, including approximately 10 minutes at peak temperature and theremaining time ramping up to and down from the peak temperature, inorder to permanently fix resistive traces 164 in position. Conductivetraces 166 and 174 a, 174 b are then printed on ceramic substrate 160,which includes selectively applying a paste containing conductormaterial in the same manner as the resistor material. The ceramicsubstrate 160 having the printed resistor and conductor is then allowedto settle, dried and fired in the same manner as discussed above withrespect to resistive traces 164 in order to permanently fix conductivetraces 166 and 174 a, 174 b in position. Glass layer(s) 180 are thenprinted in substantially the same manner as the resistors andconductors, including allowing the glass layer(s) 180 to settle as wellas drying and firing the glass layer(s) 180. In one embodiment, glasslayer(s) 180 are fired at a peak temperature of approximately 810degrees Celsius, slightly lower than the resistors and conductors.Thermistor 172 is then mounted to ceramic substrate 160 in a finishingoperation with the terminals of thermistor 172 directly welded toconductive traces 174 a, 174 b.

Thick film printing resistive traces 164 and conductive traces 166 onfired ceramic substrate 160 provides more uniform resistive andconductive traces in comparison with conventional ceramic heaters, whichinclude resistive and conductive traces printed on green state ceramic.The improved uniformity of resistive traces 164 and conductive traces166 provides more uniform heating across contact surface 118 as well asmore predictable heating of heater 130.

Preferably, heaters 130/132 are produced in an array for costefficiency. Heaters are separated into individual heaters 130/132 afterthe construction of all heaters is completed, including firing of allcomponents and any applicable finishing operations. In some embodiments,individual heaters are separated from the array by way of fiber laserscribing. Fiber laser scribing tends to provide a more uniformsingulation surface having fewer microcracks along the separated edge incomparison with conventional carbon dioxide laser scribing.

It will be appreciated that the example embodiments illustrated anddiscussed above are not exhaustive and that the heater of the presentdisclosure may include resistive and conductive traces in many differentgeometries, including resistive traces on the outer face and/or theinner face of the heater, as desired. Other components (e.g., athermistor) may be positioned on either the outer face or the inner faceof the heater as desired.

The present disclosure does, however, provide a ceramic heater having alow thermal mass in comparison with the heaters of conventional hairirons. In particular, thick film printed resistive traces on an exteriorface (outer or inner) of the ceramic substrate provides reduced thermalmass in comparison with resistive traces positioned internally betweenmultiple sheets of ceramic. The use of a thin film, thermally conductivesleeve, such as a polyimide sleeve) also provides reduced thermal massin comparison with metal holders, guides, etc. The low thermal mass ofthe ceramic heater of the present disclosure allows the heater, in someembodiments, to heat to an effective temperature for use in a matter ofseconds (e.g., less than 5 seconds), significantly faster thanconventional hair irons. The low thermal mass of the ceramic heater ofthe present disclosure also allows the heater, in some embodiments, tocool to a safe temperature after use in a matter of seconds (e.g., lessthan 5 seconds), again, significantly faster than conventional hairirons.

Further, embodiments of the hair iron of the present disclosure operateat a more precise and more uniform temperature than conventional hairirons because of the closed loop temperature control provided by thethermistor in combination with the relatively uniform thick film printedresistive and conductive traces. The low thermal mass of the ceramicheater and improved temperature control permit greater energy efficiencyin comparison with conventional hair irons. The rapid warmup andcooldown times of the ceramic heater of the present disclosure alsoprovide increased safety by reducing the amount of time the hair iron ishot but unused. The improved temperature control and temperatureuniformity further increase safety by reducing the occurrence ofoverheating. The improved temperature control and temperature uniformityalso improve the performance of the hair iron of the present disclosure.

Referring now to FIG. 6A, a method 185 is provided for determining theAC line voltage. At 187, both the heaters (130, 132) are set to operateat a given setpoint temperature (T0°) well underneath their operatingtemperature. If the operating temperature is 200° C., for example, thena representative setpoint temperature is about 150° C. At 189, thethermistors (172) read the initial or current temperatures (T1°) of theheaters and at 191 a corresponding timer is initialized (t0) and beginsits count. At the same time, at 193, current is allowed to heat theheaters to begin heating up to their setpoint temperatures, but in amanner whereby only 50% of the power is turned on. This also occurs in amanner whereby the controller only allows half-cycles of the AC linevoltage to heat the heaters, say five out of ten half-cycles, or 50%. Inthis way, the controller keeps the heaters from overheating and preventscracking. At 195, the controller determines whether the currenttemperature T1° is less than or equal to the setpoint temperature T0°less 20° C. (i.e., is T1°≤T0°−20° C.?). If so, the warmup time (WUT) ofeach heater is calculated at 199. If not, the current temperature of theheater is already much closer to the setpoint temperature and so theline voltage that powers the hair iron is calculated in addition tocalculating the warmup time of the heaters at 197. At 201, thecontroller continues monitoring of the temperature of the heaters and atime or timer count (t1) is determined when the current temperature isten degrees more than initially read (i.e., T1°+10° C.). Similarly, thecontroller monitors the temperature of the heaters at 203 and the timeor timer count (t2) is noted when the current temperature is twentydegrees more than initially read (i.e., T1°+20° C.). Then, at 205, thewarmup time (WUT) of a given heater is the time or timer countdifference between t2 and t1 (i.e., t2-t1).

Once known, the controller operates the heaters under a scheme of eitherlow or high voltage control at 207. Intuitively, if the line voltagecorresponds to a low power source, such as 115 VAC, the time to heat theheaters from their setpoint will be slower than if the line voltagecorresponds to a high-power source, such as 230 VAC, and vice versa. Itis expected that low voltage control occurs for AC line voltages between90-140 VAC and high voltage control occurs above that. In a furtherembodiment, if the warmup time t2−t1 is less than or equal to 255 msec(t2−t1≤255 milliseconds), a line voltage bit is set to “1” for highvoltage temperature control. Conversely, if the warmup time t2−t1 isgreater than 334 msec (t2−t1>334 milliseconds), the line voltage bit isset to 0 for low voltage temperature control. The below Table shows thispictorially.

Heater Power Current Measured WUT limit Voltage Bit WUT > 334 msec WUT <255 msec 50% 0\LV Take No Action Reset voltage bit to 1 50% 1\HV Resetvoltage bit to 0 Take No Action

After finishing line voltage detection 185, the AC manager of thecontroller will immediately switch from line voltage detection to actualcontrol of the temperature of the heaters 130, 132. Based on thetemperature difference of the measured heater temperature by thethermistor and the setpoint temperature, the PID(proportional-integral-derivative) controller will calculate atemperature response to the current temperature to set the requiredheating power for each heater. The controller will adjust PID gains in amanner to minimize warm up time, reduce ramp up temperature overshoot,and achieve tight steady state temperature control. In general, however,if the current heater temperature is more than 30° C. lower than thesetpoint temperature, ramping up PID gains will be used. Otherwise,steady state PID gains will be selected.

Referring now to FIG. 6B, an example method 200 for controlling powerdelivered to the heaters 130, 132 is illustrated. In it, a determination202 of the line voltage corresponds to either low power, such as 115VAC, or high power, such as 230 VAC. If the voltage is low power, thereexists no special power restrictions 204 on the heaters 130, 132 andflicker does not represent a sizable concern. If high power exists, onthe other hand at 206, the heating power to the heaters is capped at 25%capacity. Once there, the PID output power or temperature response isdetermined. If the PID output power is less than 10% at 208, meaning thecurrent temperature is relatively close to the desired temperature ofthe heaters, then the heaters are turned off at 210. If the PID outputpower at 208 is equal to or more than 10%, but less than 20%, theheaters 130, 132 are multiplexed (MUX) together in a manner at 212whereby their heater power turned on for four of twenty-four half-cyclesof AC power, or 4/24=16.7% (FIG. 7A, described below). If the PID outputpower at 208 is greater than or equal to 20%, the heaters 130, 132 aremultiplexed together in a manner at 214 whereby their heater power isturned on for six of twenty-four half-cycles of AC power, or 6/24=25%(FIG. 7B, described below).

With reference to FIGS. 7A and 7B, when situations dictate that theheaters 130, 132 become multiplexed, neither heater is powered on at thesame time as the other heater and powering on and off occurs atzero-crossings of the AC waveform to reduce flicker and harmoniccomponents. In either situation, the waveform of the AC line voltage isdivided into 24 half-cycles of AC power (numbered 1-24). A first twelveof the 24 half-cycles are reserved for one heater while the last twelvecycles are reserved for the other heater. Line 250 demarks the first andlast twelve cycles from one another. Thereafter, in a repeating pattern,current becomes applied to the first heater 130 during the first twelvecycles while current is next applied to the opposite heater 132 duringthe next twelve cycles, or vice versa. As seen, solid lines 260-x, 270-yrepresent the application of current to the heaters thereby poweringthem on, whereas the dashed lines correspond to no application ofcurrent to the heaters. Thus, heater 132 is off at 264 during the secondtwelve half-cycles of AC power when heater 130 is powered on at duringthe first twelve half-cycles of AC power. Conversely, heater 130 is offat 262 during the first twelve half-cycles of AC power when heater 132is powered on at during the second twelve half-cycles of AC power. Thateither heater in FIG. 7A is only powered on for four of the twenty-fourhalf-cycles of AC power (260-1, 260-2, 260-3, 260-4 corresponding toheater 130 and 260-5, 260-6, 260-7, 260-8 corresponding to heater 132),only 16.7% power levels of the heaters are achieved in the hair iron, or4/24=16.7%. Similarly, in FIG. 7B, that either heater is only powered onfor six half-cycles of the twenty-four half-cycles of AC power (270-1,270-2, 270-3, 270-4, 270-5, 270-6 corresponding to heater 130 and 270-7,270-8, 270-9, 270-10, 270-11, 270-12 corresponding to heater 132), only25% power levels of the heaters are achieved in the hair iron, or6/24=25%. Of course, other percentages of power levels are possible, asare different half-cycles of AC power.

As examples, with reference back to FIG. 3 , artisans will note that theheaters 130, 132 are two independent heating elements of equalresistance and each has a current temperature feedback mechanism by wayof the thermistor 172 to the controller 135. During use, the controlleractivates the switch 127 to control AC power delivery to the heaters.Using the AC zero-crossing feedback 129, the power delivery issynchronized precisely with the zero-crossings of the AC mains voltagewaveform. This establishes the minimum unit of power delivery as asingle half-cycle of the AC waveform. The controller modulates thecurrent of each heater to achieve a desired temperature. This action ismoderated by a temperature control loop (e.g. PID) running on thecontroller. That is, the control loop calculates a desired temperatureresponse by way of a power level in units of percent, where 100% isequal to rated wattage of the heater. The fundamental period of heaterpower delivery in the following embodiments is eight half-cycles.

With reference to the table 300 of FIG. 8 , the controller will causethe switch to connect heaters to the AC line voltage for an integernumber of half-cycles within this period. To achieve a power levelpercent (%) of 12.5% (e.g., 1/8×100%), for example, one AC half-cycle302 of one-thru-eight total half-cycles 301 of power is turned on toheat the heater. Similarly, to achieve a power level percent of 50%,four AC half-cycles 304 of one-thru-eight total half-cycles 301 of powerare turned on to the heat the heater (e.g., 4/8×100%). Similarly, too,all power level percentages of the heaters is read from the table 300,e.g., power level percentages 0%, 12.5%, 25%, 37.5%, 50%, 62.5%, 75%,87.5%, and 100%. However, depending on the exact number and placement ofON half-cycles in the one-thru-eight half-cycle period (e.g., the“pattern”), there arises a DC offset in the resultant current waveform.Namely, if there exists an odd number 1, 3, 5, and 7 of AC half-cyclesof power turned ON in the one-thru-eight AC half-cycles of power,corresponding to power level percentages 12.5%, 37.5%, 62.5%, and 87.5%,respectively, a DC offset 306, 308, 310, and 312 results, respectively.Conversely, if the number of positive-polarity half-cycles equals thenumber of negative-polarity half-cycles, or there exists zero or an evennumber 0, 2, 4, 6, and 8, of AC half-cycles of power turned ON in theone-thru-eight AC half-cycles of power, corresponding to power levelpercentages 0%, 25%, 50%, 75%, and 100%, respectively, a DC offset doesnot exist. Stated mathematically, the DC offset 320 for a given patterncan be quantified by taking the difference between positive-polarity andnegative-polarity ON half-cycles, or DC Offset=Number of positive AChalf-cycles−Number of negative AC half-cycles. Importantly, theinventors observe that when the total length of the pattern is an evennumber of AC half-cycles, if the pattern is reversed the patternsequence will yield a new pattern whose DC offset is equal in magnitudeyet opposite in sign. This is because positive half-cycles will bereplaced by negative half-cycles and vice versa. In turn, three Cases 1,2, and 3 arise whereby the PID controllers calculate a desiredtemperature response for its given heater whereby the power levelpercent to be applied to the heaters has both power level percentageswith a DC offset of 1 (Case 1); has both power level percentages with aDC offset of 0 (Case 2); and has one power level percent for one heaterwith a DC offset of 1 and one power level percent for the other heaterwith a DC offset of 0 (Case 3).

For Case 1, with further reference to FIG. 9A, if a desired temperatureresponse calculated by the PID controller for the heaters 130, 132 isboth a power level percent having a DC offset, such as power level37.5%, where there exists three ON AC half-cycles of power, thecontroller switches the AC half-cycles of power to the heaters 130, 132in a manner that results in a combined DC offset of 330 of zero. Thatis, there exists a first 332 AC half-cycle of power of 37.5% applied tothe heater 130 that is a reversed pattern of a second 333 AC half-cycleof power of 37.5% applied to the heater 132. As the former has a DCoffset 335 of +1, while the latter has a DC offset 337 of −1, thereexists zero DC offset 330 applied to the combined, both heaters 130,132. This leverages the fact that the resistances of the heaters 130,132 are equal to one another and therefor the current amplitude to bothheaters is equal.

For Case 2, if a given a desired temperature response calculated by thePID controller for the heaters 130, 132 is both a power level percenthaving no DC offset, or zero DC current, a trivial case exists and thecontroller merely switches the AC half-cycles of power to the heaters130, 132 according to the table 300.

For Case 3, the situation exists whereby a desired temperature responsecalculated by the PID controller for the heaters 130, 132 has both a DCoffset for one of the heaters 130 or 132, but not both of the heaters130 and 132. If summing together the two AC waveforms cannot result in acombined DC offset of zero, such as at 330 (FIG. 9A), FIG. 9B teachesthe extension of the total pattern period of AC half-cycles to sixteen(16) AC half-cycles, instead of eight. In turn, the first eight AChalf-cycles 340, the AC waveform pattern applied to one of the heatersis provided in the forward direction and the second eight AC half-cycles342 is provided to the other or opposite heater is in the reversedirection, or vice versa. Since the forward and reverse of a givenpattern have equal and opposite DC offsets, the complete waveform haszero DC bias 344 over the sixteen half-cycle period. Of course, thesethree cases could be changed to more or fewer AC half-cycles of power.

Advantages should be now readily apparent to those skilled in the art.Among them, the hair iron herein: 1) can be conveniently used anywherein the world without needing a voltage converter; 2) successfully meetsIEC flicker and harmonic requirements, while many competitive productscannot; and 3) delivers thermal performance that readies itself in asfew as five seconds, with fast cooling times.

The foregoing description illustrates various aspects of the presentdisclosure. It is not intended to be exhaustive. Rather, it is chosen toillustrate the principles of the present disclosure and its practicalapplication to enable one of ordinary skill in the art to utilize thepresent disclosure, including its various modifications that naturallyfollow. All modifications and variations are contemplated within thescope of the present disclosure as determined by the appended claims.Relatively apparent modifications include combining one or more featuresof various embodiments with features of other embodiments.

1. A hair iron, comprising: two longitudinally extending arms eachhaving a heater corresponding to an opposite heater on an opposite armof the two longitudinally extending arms, wherein during use hairbecomes placed between said heater and opposite heater to heat the hair,said heater and opposite heater being a ceramic heater and having acorresponding thermistor; and a controller to coordinate heating of saidheater and opposite heater by connecting or not said heater and oppositeheater to an AC line voltage at times of zero crossings of a waveform ofthe AC line voltage and to receive from the corresponding thermistor acurrent temperature of said heater and opposite heater to calculate adesired heating power.
 2. The hair iron of claim 1, further including aTRIAC to said connect or not said heater and opposite heater to the ACline voltage.
 3. The hair iron of claim 1, further wherein thecontroller includes a PID controller corresponding to said heater andopposite heater and connected to the corresponding thermistor.
 4. Thehair iron of claim 3, wherein the controller is further configured toalternate heating between said heater and opposite heater and heatingonly one of said heater and opposite heater at any time.
 5. The hairiron of claim 3, wherein the desired heating power corresponds to awaveform of the AC line voltage and having or not a DC offset dependingupon a number of even or odd on-and-off half-cycles of the waveform ofthe AC line voltage, the even on-and-off half-cycles having no DC offsetbut the odd on-and-off half-cycles having said DC offset.
 6. The hairiron of claim 5, wherein if each said desired heating power of saidheater and opposite heater has no said DC offset, the controllerapplying said each said desired heating power.
 7. The hair iron of claim5, wherein if each said desired heating power of said heater andopposite heater have said DC offset, the controller applying saiddesired heating power for said heater in an opposite pattern of applyingsaid desired heating power for said opposite heater.
 8. The hair iron ofclaim 7, wherein the controller applies said desired heating power forsaid heater and opposite heater over eight (8) AC half-cycles of the ACline voltage.
 9. The hair iron of claim 5, wherein if one but not bothof said desired heating power of said heater and opposite heater hassaid DC offset, the controller applying said desired heating power forsaid heater in an opposite pattern of applying said desired heatingpower for said opposite heater.
 10. The hair iron 9, wherein thecontroller applies said desired heating power for said heater andopposite heater over sixteen (16) AC half-cycles of the AC line voltage.11. A hair iron, comprising: two longitudinally extending arms eachhaving a heater corresponding to an opposite heater on an opposite armof the two longitudinally extending arms, wherein during use hairbecomes placed between said heater and opposite heater to heat the hair,said heater and opposite heater being a ceramic heater and having acorresponding thermistor; and a controller to coordinate heating of saidheater and opposite heater by connecting or not said heater and oppositeheater to an AC line voltage at times of zero crossings of a waveform ofthe AC line voltage, wherein the controller coordinates timing so thatthe heater and opposite heater have no DC offset over a range of AChalf-cycles of the waveform.
 12. The hair iron of claim 11, furtherincluding a TRIAC to said connect or not said heater and opposite heaterto the AC line voltage.
 13. The hair iron of claim 11, further whereinthe controller includes a PID controller corresponding to said heaterand opposite heater and connected to the corresponding thermistor. 14.The hair iron of claim 13, wherein the controller is further configuredto alternate heating between said heater and opposite heater and heatingonly one of said heater and opposite heater at any time.
 15. The hairiron of claim 11, wherein the range of AC half-cycles of the waveform iseight (8) AC half-cycles.
 16. The hair iron of claim 11, wherein therange of AC half-cycles of the waveform is sixteen (16) AC half-cycles.17. The hair iron of claim 11, wherein the controller is furtherconfigured to receive from the corresponding thermistor a currenttemperature of said heater and opposite heater to calculate a desiredheating power.
 18. The hair iron of claim 17, wherein the desiredheating power includes desired AC half-cycles of power being on for 0,25, 50, 75 and 100 percent (%) of the desired AC half-cycles.
 19. Thehair iron of claim 18, further including said desired AC half-cycles ofpower being on for 12.5, 37.5, 62.5 and 87.5 percent (%).
 20. A hairiron, comprising: two longitudinally extending arms each having a heatercorresponding to an opposite heater on an opposite arm of the twolongitudinally extending arms, wherein during use hair becomes placedbetween said heater and opposite heater to heat the hair, said heaterand opposite heater having a corresponding thermistor; and a controllerto coordinate heating of said heater and opposite heater by connectingor not said heater and opposite heater to an AC line voltage at times ofzero crossings of a waveform of the AC line voltage, wherein thecontroller coordinates timing so that the heater and opposite heaterhave no DC offset over a range of AC half-cycles of the waveform andalternates heating between said heater and opposite heater such thatonly one of said heater and opposite heater are powered at any one time.